When it came to overclocking the Phenom II X4 965, I aimed high trying to get over 4.0GHz on air. For this I used a Thermaltake ISGC-400 CPU cooler with the fan speed set on high. Even with this the temperatures soared over the 62C threshold for the CPU. With the voltage at 1.52v, which is a bit over the specified maximum, I was only able to push the processor to 3.914GHz before the system became too unstable to run benchmarks.

Even if you do manage 4Ghz your real world gains will be minimal, not even noticeable.